Fullerenes are the class of carbon compounds distinguished by their multi-faceted, closed molecular structure. The nature of the electron "shells," or orbitals, that surround the nucleus of every atom dictates that each orbital is "filled" when it contains a certain number of electrons. Atoms bond to form molecules because by bonding they can share electrons and fill shells that would otherwise be only partially filled. An unbonded carbon atom has four electrons in its outermost shell, but would prefer to have eight. For this reason, carbon atoms bond readily with other atoms, including other carbon atoms.
Under certain conditions, carbon atoms bond together such that the carbon--carbon bonds form a framework of hexagons and pentagons that resembles the familiar hexagon/pentagon surface of a soccer ball. Molecules having this structure have come to be known as fullerenes. The number and positioning of the hexagons and pentagons can vary, within both constraints that exactly 12 pentagons and that an even number of carbon atoms be present. It happens that the spherical molecule formed by sixty carbon atoms (C60) comprises a particularly stable combination of hexagons and pentagons and is the most widely studied fullerene to date. In general, more than one arrangement of the hexagons and pentagons is possible, leading to a great variety of possible isomers for any particular number of carbon atoms in a fullerene. To help specify a particular fullerene isomer, the symmetry group name to which that isomer belongs is affixed to the molecular formula, but even this is imperfect as it is common for many isomers belonging to the same point group to be present for any particular number of carbon atoms.
The currently known methods for making fullerenes involve evaporating carbon atoms and cooling them slowly, so that some of them assemble into fullerene molecules. Even under optimal conditions, however, not all of the evaporated carbon atoms end up in fullerene molecules (the remainder forms soot). While C.sub.60 -I.sub.h forms a significant fraction of the total fullerene production, the fullerene molecules that are produced can have more than three hundred carbon atoms. Under current practices, the fullerenes are extracted from the soot using a nonpolar solvent, such as toluene. C.sub.60 -I.sub.h dissolves readily in such solvents, as do several other fullerenes with isolated pentagons, including but not limited to, C.sub.70 -D.sub.5h, C.sub.76 -D.sub.2, C.sub.78 -C.sub.2v ', C.sub.78 -C.sub.2v ", C.sub.78 -D.sub.3, C.sub.80 -D.sub.2, C.sub.84 -D.sub.2d and C.sub.84 -D.sub.2. Once dissolved, the fullerenes can be recovered in a relatively pure form. Additionally, there are many empty fullerene structures that are predicted to be stable, but which are not found among the fullerenes that are extracted using the method described above. These include, but are not limited to, isolated pentagon isomers of C.sub.74 (D.sub.3h) and many larger fullerenes, such as C.sub.78 -D.sub.3h ' and C.sub.80 -I.sub.h.
Fullerenes can also be produced with one or more atoms of another material trapped inside the cage formed by the fullerene molecule. When the trapped atom is a metal, the molecule may be called a metallofullerene or endohedral metallofullerene. While various attempts have been made to produce endohedral metallofullerenes, with one exception, the only endohedral metallofullerenes that have been recovered are those containing metal atoms that have an even total number of valence electrons. For example, endohedral metallofullerenes containing Group II metals (calcium, strontium, and barium) have been isolated. Also, endohedral metallofullerenes containing two Group III metals (scandium, yttrium, and the lanthanides) have been recovered. However, endohedral metallofullerenes containing a metal atom(s) that has an odd number of valence electrons (one or three Group III atoms, e.g.) are, in general, not recoverable. The aforementioned exception occurs when the fullerene cage has 82 carbon atoms. Because they have never been recovered or isolated, the very existence of other endohedral metallofullerenes as stable, recoverable molecules has not been considered certain.
Hence, it is desirable to provide a method for recovering these previously unrecoverable fullerenes and metallofullerenes. It is further desired to provide a fullerene isolation method that is simple and easy to execute, and that does not disrupt or affect the subject fullerenes.